Sodium carbonate (Na2CO3), or soda ash, is one of the largest volume alkali commodities made worldwide with a total production in 2011 of 53 million tons. Sodium carbonate finds major use in the glass, chemicals, detergents industries, and also in the sodium bicarbonate production industry. The main processes for sodium carbonate production are the Solvay ammonia synthetic process, the ammonium chloride process, and sodium carbonate or bicarbonate ore-based processes.
Main exploited sodium carbonate or bicarbonate ores are mainly trona ore (Na2CO3.NaHCO3.2H2O), nahcolite ore (NaHCO3) or natron ore (sodium carbonate decahydrate: Na2CO3.10H2O). Most of those ore deposits may comprise also some Wegsheiderite (Na2CO3.3NaHCO3).
Trona ore is a mineral that contains up to 99% sodium sesquicarbonate (Na2CO3.NaHCO3.2H2O). Trona-based soda ash is obtained from trona ore deposits in Green River (Wyoming), Turkey, China, and Kenya either by underground mechanical mining, or by solution mining, or by lake waters processing. The trona-based sodium carbonate from Wyoming comprised about 90% of the total U.S. soda ash production.
A typical analysis of the trona ore in Green River is as follows:
TABLE 1ConstituentWeight PercentNa2CO343.4NaHCO334.4H2O (crystalline and free moisture)15.4NaCl0.01Na2SO40.01Fe2O30.14Insolubles6.3Organics0.3
Nahcolite ore is a mineral that contains up to 99% sodium bicarbonate (NaHCO3). Nahcolite-based soda ash has been produced from nahcolite ore deposits in Piceance Creek (Colorado).
Sodium carbonate or bicarbonate ore deposits include beds of corresponding sodium carbonate or bicarbonate ores listed above, and include also halite (NaCl), and/or thenardite (Na2SO4), and/or glauberite (Na2SO4.CaSO4) included within the sodium carbonate or bicarbonate ores beds or included in inter-beds layers. The quality of the ore and of its content in impurities varies, depending on its particular location in the stratum and according geographic location. Aside sodium chloride, and/or sodium sulfate, the deposits contain other soluble impurities such as potassium chloride, potassium sulfate, alkaline metal borate, and alkaline metal phosphates, and slightly soluble minerals such as: alkali metal and alkali earth metal silicates, aluminates, titanates, vanadates, metallic compounds and salts.
Sodium carbonate or bicarbonate ores comprise also organic impurities. Such impurities come from organic sediments that were captured during the formation of the deposits and that frequently have formed oil shales during geological aging. Those organic impurities may be present in the Sodium carbonate or bicarbonate ore and/or also present in inter-beds of the ore deposits.
Other “insoluble” or very slightly water-soluble mineral impurities found in sodium carbonate or bicarbonate ores deposits or in the shales beds are generally mixtures of different minerals; the most frequent of which are calcite, dolomite, pirssonite, zeolite, feldspar, clay minerals, iron/aluminium silicates, and calcium sulfate.
Two main techniques well known in the art are used to recover sodium carbonate or bicarbonate ores from ore deposits. The first technique is mechanical mining, also called conventional mining, such as a room and pillar panel operation or a longwall operation. The second technique is a solution mining recovering wherein the ore, such as trona or nahcolite, is dissolved with water and recovered as a solution.
Among the several ways in which sodium carbonate can be recovered from sodium carbonate or bicarbonate ores that contains other salts and impurities, the most widely practiced is the so called “monohydrate process”. In that process a mined ore, such as trona ore, is crushed, then calcined into crude sodium carbonate, then leached with water, the resulting water solution is purified and fed to a crystallizer where pure sodium carbonate monohydrate crystals are crystallized. The monohydrate crystals are separated from the mother liquor and then dried into anhydrous sodium carbonate. Most of the mother liquor is recycled into the crystallizer. However, the soluble impurities contained in the ore, tend to accumulate in the mother liquor in the crystallizer. To avoid build up of soluble impurities, the mother liquor must be purged. The purge liquor, which represents important quantities for industrial monohydrate plants, is commonly sent to an evaporative pond, also called tailings pond. The significant quantity of alkali which is contained in the purge liquor is consequently lost. Moreover, the stocking of large quantities of purge liquors in evaporative ponds raise environmental problems, because of the scarce availability of new areas for stocking, and induces also water loss that is detrimental in arid regions.
Variants to produce sodium carbonate from a solution generated with sodium carbonate or bicarbonate ores, in particular from solution mining, are:                decomposing thermally (with steam) or calcine chemically (with caustic soda) the dissolved sodium bicarbonate of the solution to transform it into dissolved sodium carbonate, then evaporating the water in order to crystallize pure sodium carbonate monohydrate,        or crystallizing refined sodium sesquicarbonate (sesqui) after adjusting sodium bicarbonate to sodium carbonate molar ratio of the solution, evaporating part of water, then calcining the refined sesqui into soda ash,        or crystallizing refined sodium bicarbonate after adjusting sodium bicarbonate to sodium carbonate molar ratio of the solution using carbon dioxide, then calcining the refined sodium bicarbonate into soda ash.        
In those variants, the soluble impurities contained in the sodium carbonate or bicarbonate ore, tend to accumulate also into the monohydrate, or the sesqui or the sodium bicarbonate crystallizers. To avoid the build up of impurities, the mother liquors must also be purged, raising the same environmental problems in evaporative ponds as the monohydrate process.
Sodium bicarbonate (NaHCO3), aside sodium carbonate, is another important alkali product with a wide range of applications including human food, animal feed, flue gas treatment, and chemical industries. The production of sodium bicarbonate is currently almost entirely made by the carbonation of solid or aqueous solutions of sodium carbonate with gaseous CO2 either produced in situ in the soda ash plants or purchased independently.
Several technical alternatives have been proposed to reduce the purge volume from soda ash plants.
U.S. Pat. No. 3,184,287 discloses a method for producing soda ash from trona, preventing the formation of an insoluble dissolution barrier on the face of underground trona deposit subjected to in situ solution mining, wherein a portion of a stream liquor from a sodium carbonate monohydrate crystallizer is mixed with slaked lime in a causticizer to produce an aqueous sodium hydroxide comprising 3 to 10% of NaOH which is recycled back to the in-situ dissolution of trona deposit to produce a sodium carbonate solution.
US2003/0143149 discloses a process for recovering sodium-based chemicals from sodium carbonate streams such as recycle, purge, and waste streams from sodium carbonate crystallizers, mine water, evaporative pond water and sodium carbonate decahydrate deposits. The sodium bicarbonate concentration from those streams is reduced into sodium carbonate by decarbonizing such streams with quick lime (CaO) or sodium hydroxide (NaOH) and the resulting stream is fed mainly back to a sodium carbonate monohydrate crystallizer, and the remainder of the resulting decarbonized stream is fed to a sodium carbonate decahydrate crystallizer, from which purified decahydrate is recovered and recycled to monohydrate crystallizer and a purge concentrated in impurity such as sodium sulfate is disposed off. Though the purge reduction factor of this process is limited, because when high concentration of impurities is reached, sodium carbonate and sodium sulfate forms decahydrated mixed salts. And if high amounts of sodium sulfate are recycled back to carbonate monohydrate crystallizer, they generate burkeite crystals (Na2CO3.2Na2SO4) that are detrimental to sodium carbonate monohydrate quality.
US2004/0057892 discloses a process for the production of sodium carbonate and bicarbonate, according to which a purge liquor from a monohydrate sodium carbonate crystallizer is introduced into a sodium carbonate decahydrate crystallizer and the purified decahydrate crystals are converted into sodium bicarbonate. It has been observed that this process is not efficient when the purge liquor, depending on the trona source, contains high levels of impurities. High level of sodium chloride in the purge liquor prevents smooth crystallization of sodium carbonate decahydrate, displacing the sodium carbonate heptahydrate and sodium sesqui concentration domains.
U.S. Pat. No. 7,507,388 discloses a process for the production of sodium carbonate and bicarbonate, from a pre-purified solution comprising bicarbonate which is first partially decarbonized and then used in both a sodium bicarbonate line and a sodium carbonate monohydrate line. The purge stream of the sodium carbonate monohydrate crystallizer is either sent into a mixed sodium carbonate decahydrate and sodium sesquicarbonate line wherein resulting filtrate is discarded as the final purge of the process or sent after dilution into a light soda ash line comprising an intermediate sodium bicarbonate carbonation step, the bicarbonate is separated from the filtrate, and this filtrate is also disposed as a final purge. The taught total amounts of generated purges (in streams 322 and 422) is 1.28 t of purges per ton of dense soda ash and corresponds to 6 to 15 weight percents of purged sodium carbonate per ton of produced dense soda ash, which represents considerable sodium carbonate loss.
US2009/0291038 (Solvay) discloses a process for the joint production of sodium carbonate and sodium bicarbonate crystals, according to which a solid powder derived from sodium sesquicarbonate such as calcined trona is dissolved in water, the resulting water solution is introduced into a crystallizer, wherein sodium carbonate crystals and a mother liquor are produced, part of the mother liquor is taken out of the crystallizer (purge of the sodium carbonate crystallizer) and is carbonized (carbonated) to produce valuable sodium bicarbonate crystals and a second mother liquor, the second mother liquor is optionally decarbonized (debicarbonated) and then sent to a storage pond or a tailings pond.
However, there is still a need in the sodium carbonate and bicarbonate industry, to be able to reduce impurities build-up and further reduce the purge volume and the loss of alkali in a simple way, without impairing operation conditions of the linked processes.